1. Field of the Invention
The present invention relates to a Radio Frequency (RF) amplifier in a communication system and a method of controlling the RF amplifier. More particularly, the present invention relates to an RF amplifier for performing a fine gain control and a method of controlling the RF amplifier performing the fine gain control.
2. Description of the Related Art
An amplifier should provide a gain that is suitable for a signal received through an antenna in a wireless communication system, and a Low Noise Amplifier (LNA) is commonly used to provide a suitable gain.
The low noise amplifier may be an amplifier for amplifying an RF signal received through an antenna in the wireless communication system, and the low noise amplifier amplifies the received signal so that a desired Radio Frequency (RF) signal may be obtained while generating little noise in the desired RF signal. In order to obtain the desired RF signal, the low noise amplifier should provide sufficient variable gains. Hereinafter, a method of providing a variable gain in the low noise amplifier in the related art will be described with reference to FIGS. 1 to 3.
FIG. 1 schematically illustrates an internal structure of a low noise amplifier in a communication system according to the related art.
Referring to FIG. 1, the low noise amplifier includes an input unit 110, a cascode unit 120, and a load unit 130.
Differential RF signals, such as a positive (+) signal and a negative (−) are received by the input unit 110 through an antenna (not shown) and are amplified according to an impedance value of the load unit 130, and the amplified differential RF signals (+, −) are output through the cascode unit 120. That is, input terminals 101 and 103 respectively receive one of the differential input signals of the low noise amplifier, and output terminal 141 and 143 output the amplified differential input signals. Hereinafter, it should be understood that “+” and “−” in an input side correspond to the input terminals and “+” and “−” in an output side correspond to the output terminals.
As described above, in order to obtain a desired RF signal from the received RF signal, the low noise amplifier should include sufficient variable gains, and a gain control for providing the sufficient variable gains may be achieved through modifications in a circuit of the cascode unit 120 or the load unit 130, as described in more detail with reference to FIGS. 2 and 3.
FIG. 2 is a circuit diagram for describing a gain control method using a cascode unit according to the related art.
Referring to FIG. 2, an example of the cascode unit 120 of the low noise amplifier of FIG. 1 is shown in more detail. The low noise amplifier for performing the gain control through the cascode unit 120 includes a transistor terminal 121 for performing an amplification operation according to a bias voltage (VBias) connected and applied to the input unit 110 and the load unit 130, a first gain controller 122a connected to an end of the transistor terminal 121, and a second gain controller 122b connected to another end of the transistor terminal 121.
Each of the first and second gain controllers 122a and 122b includes N transistors, TRGC0 to TRGC(N-1), involved in the gain control. In the circuit diagram shown in FIG. 2, the RF signal is transferred to one or more transistors selected according to a control of an external circuit (not shown), and accordingly the gain is controlled, and thus a size of the RF output signal may be determined.
FIG. 3 is a circuit diagram for describing a gain control method through a load unit according to the related art.
Referring to FIG. 3, the load unit 130 of the low noise amplifier of FIG. 1 is shown in more detail. The low noise amplifier for performing the gain control through the load unit 130 includes a third gain controller 132a connected to an end of the cascode unit 120 to control the gain of the RF signal and also includes a fourth gain controller 132b connected to another end of the cascode unit 120 to control the gain of the RF signal. Each of the third and fourth gain controllers 132a and 132b includes N transistors, TRGC0 to TRGC(N-1), involved in the gain control and resistors, R0 to R(N-1), each of the resistors being respectively connected to an end of each of the transistors TRGC0 to TRGC(N-1).
In the circuit diagram shown in FIG. 3, the RF signal is transferred to at least one transistor selected according to a control of an external circuit (not shown), and the gain is controlled according to a resistance value of the respective resistance connected to an end of the at least one selected transistor, and thus a size of the RF output signal may be determined. The gain control methods in the related art, as described with reference to FIGS. 2 and 3, use N−1 additional transistors for the gain control of N steps. Accordingly, parasitic components corresponding to a number of increased transistors may be generated, and the generated parasitic components may result in deterioration of frequency characteristics, noise characteristics, linearity and other characteristics of an RF amplifier.
Accordingly, a need exists for a system and a method of controlling a gain while not generating a deterioration of capabilities of an RF amplifier.